As is known in the art, a phased array antenna includes a plurality of antenna elements spaced apart from each other by known distances. Each of the antenna elements are typically coupled through a plurality of phase shifter circuits, amplifier circuits and/or other circuits to either or both of a transmitter or receiver. In some cases, the phase shifter, amplifier circuits and other circuits (e.g. mixer circuits) are provided in a so-called transmit/receive (T/R) module and are considered to be part of the transmitter and/or receiver.
The phase shifters, amplifier and other radio frequency (RF) circuits (e.g. T/R modules) are often powered by an external power supply (e.g. a DC power supply). Such circuits are referred to as “active circuits” or “active components.” Accordingly, phased array antennas which include active circuits are often referred to as “active phased arrays.”
Active circuits dissipate power in the form of heat. Thus, it is necessary to cool active phased arrays so that the active circuits operate within a desired temperature range.
In active phased arrays having T/R channels which use relatively little power (e.g. less than about two Watts (W) average RF power), individual finned heat-sinks (or “hat-sinks”) are sometimes attached to each active circuit in the channels. That is, each active circuit has an individual heat sink attached thereto. Although this approach may satisfy the cooling requirements for the active phased array, this approach to thermal management is expensive since the cost of disposing an individual “hat-sink” on an active circuit can be on the same order as the cost of the active circuit itself.
In relatively high power per T/R channel applications, it is often necessary to use a liquid cooling approach to maintain active circuits in their normal operating temperature range. Although the liquid cooling approach is effective to maintain active circuits at temperatures at or below maximum allowed operating temperatures, liquid cooling has very high life cycle costs. For example, liquid cooling requires the use of a manifold through which the liquid circulates. Such liquid filled manifolds add a tremendous amount of weight and complexity to a radar system which increases the radar system recurring cost and also increases the transportation costs and maintenance costs over the operational life of the active phased array.
The mechanical/thermal interfaces between the heat generating devices (e.g. the active circuits) and heat sinking devices determines, at least in part, the cooling effectiveness of heat sinking devices.
Some RF systems, including active phased arrays, utilize so-called flip-chip mounted circuits. One technique commonly used to remove heat from flip-chip mounted circuits (or more simply, “flip-chips”) is to dispose a gap-pad between the exposed surface of the flip-chip and a surface of a heat sink. In this configuration, the gap-pad needs to be compliant in compression and shear to compensate for coplanarity tolerances from chip-to-chip and in-plane movement due to coefficient of thermal expansion (CTE) mismatch between the flip-chip, circuit board and heatsink as well as vibration between the circuit board on which the flip-chip circuit is mounted and the heatsink. The gap-pad technique can result in a thermal path having poor bulk thermal conductivity. Furthermore, the gap-pad approach results in thermal junctions on each surface of the gap pad (i.e. one thermal junction between the gap-pad and the chip and one thermal junction between the gap-pad and the heatsink). Such thermal junctions would not exist if the heatsink were directly mounted to the flip-chip. Furthermore, the thermal resistance at these junctions is relatively high compared with the thermal resistance which would result if the heatsink were directly mounted to the flip-chip.
It would, therefore, be desirable to provide a reliable, efficient and cost effective system and technique for cooling RF systems including active phased arrays which operate over a wide range of RF output power levels.